Half-sandwich rhodium/iridium(III) complexes designed with Cp* and 1,2-bis(phenylchalcogenomethyl)benzene as catalysts for transfer hydrogenation in glycerol

Article abstract of DOI:10.1021/om500149n

The reactions of 1,2-bis(phenylthiomethyl)benzene(L1) and 1,2-bis(phenylselenomethyl)benzene(L2) with [(η⁵-Cp*) MCl(μ-Cl)]₂ (M = Rh or Ir) at room temperature, followed by treatment with NH₄PF₆ have resulted in

Full text of DOI:10.1021/om500149n

Article
pubs.acs.org/Organometallics
Half-Sandwich Rhodium/Iridium(III) Complexes Designed with Cp*
and 1,2-Bis(phenylchalcogenomethyl)benzene as Catalysts for
Transfer Hydrogenation in Glycerol
Om Prakash, Kamal Nayan Sharma, Hemant Joshi, Pancham L. Gupta, and Ajai K. Singh*
Department of Chemistry, Indian Institute of Technology Delhi, New Delhi 110016, India
*
S Supporting Information
ABSTRACT: The reactions of 1,2-bis(phenylthiomethyl)benzene(L1) and 1,2-
5
bis(phenylselenomethyl)benzene(L2) with [(η -Cp*)MCl(μ-Cl)] (M = Rh or Ir)
2
at room temperature, followed by treatment with NH PF have resulted in air and
moisture insensitive half-sandwich complexes of composition [(η -Cp*)M(L)Cl]-
PF ] (Rh, 1−2; Ir, 3−4; L = L1 or L2). Their HR-MS, H, C{ H}, and Se{ H}
4
6
5
1
13
1
77
1
[
6
NMR spectra were found to be characteristic. The single crystal structures of 1−4
have been established by X-ray crystallography. The complexes 1−4 have been
found efficient for catalytic transfer hydrogenation (TH) of aldehydes and ketones
in glycerol, which acts as a solvent and hydrogen source. Complexes 1−2 are the
first examples of Rh species explored for TH in glycerol. The catalysis appears to be
homogeneous. The complexes of the (Se, Se) ligand are marginally efficient than
the corresponding complexes of the (S, S) ligand. The reactivity of Rh complexes in
comparison to those of Ir also appears to be somewhat more. The results of DFT
calculations appear to be generally consistent with experimental catalytic
efficiencies and bond lengths/angles.
INTRODUCTION
this main byproduct is not going to cut the cost very
■
significantly. TH of organic carbonyl compounds requires a
catalyst. Several ruthenium species including half-sandwich9
ones have been reported to catalyze TH reactions in glycerol,
However, rhodium/iridium complexes are much less explored
The combination of an efficient catalyst and a nontoxic solvent
like glycerol is attractive. Further, edible and biodegradable
1
2
glycerol is attractive due to its low cost, ready availability, and
3
renewability, being the main byproduct in oleochemical
10
production. Several processes of the conversion of biomass to
chemicals and fuels occur via glycerol. Nonhazardous glycerol
for TH reactions, particularly in glycerol.
4
In view of our current research interest in the designing of
is also a good solvent as it dissolves inorganic salts, acids, bases,
enzymes, transition-metal complexes, and organic compounds
transition metal catalysts for reactions such as C−C coupling,
11
oxidation of alcohol, and TH using organochalcogen ligands,
it was thought worthwhile to synthesize complexes of 1,2-
bis(phenylchalcogenomethyl)benzene (L1−L2, Chart 1) with
(
poorly miscible in water). Thus, in glycerol a variety of
transformations is feasible. Hydrophobic solvents such as ethers
and hydrocarbons being immiscible with glycerol may be used
to remove products from it by simple extraction. The high
boiling point (290 °C) makes it a suitable solvent for reactions
to be carried out at a high temperature (not possible with the 2-
propanol known for its use in transfer hydrogenation (TH)).
Distillation is a feasible technique for the separation of the
products of reactions carried out in glycerol, which being a
nonflammable solvent does not require special handling or
5
η -Cp*Rh(III)/Ir(III) and explore them as TH catalysts in
Chart 1
5
storage.
TH is a convenient and versatile method for the reduction of
7
carbonyl groups of ketones and aldehydes. 2-Propanol has
been widely used as a source of hydrogen in TH. It eliminate8s
inflammable hydrogen gas and the need for pressure vessels.
Use of glycerol as a hydrogen source is successful for TH
6
reactions but has received less attention. In TH reactions,
glycerol is dehydrogenated to several products including
dihydroxyacetone. However, low yield of dihydroxyacetone is
not a big concern as glycerol is very cheap, and low recovery of
9
Received: February 10, 2014
Published: May 2, 2014
©
2014 American Chemical Society
2535
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Article
Scheme 1. Synthesis of Ligands L1−L2 and Complexes 1−4
glycerol (solvent and hydrogen source). To our knowledge,
there is no use of glycerol as a hydrogen source in TH catalyzed
with Rh/Ir(III) complexes of organochalcogen ligands. The
present Rh complexes (1−2) are the first examples of Rh
species explored for TH in glycerol. Herein we present the
results of such investigations. Complexes 1−4 have been
characterized with X-ray diffraction. The catalytic efficiencies of
the present half-sandwich complexes of Rh/Ir(III) with L1 and
L2 have been found promising. The results of density
functional theory (DFT) calculations have also been included
in this article and have been found to be consistent with several
experimental results.
bonded to metal ions in a bidentate mode in all complexes 1−
4, resulting in a seven membered chelate ring with both of
them. The molecular structure diagrams of cations of 1−4 are
given in Figures 1 to 4 with selected bond lengths and angles.
RESULTS AND DISCUSSION
■
The syntheses of L1−L2 and their complexes (1−4) have been
summarized in Scheme 1. The present procedures for the
preparation of L1 and L2 are easier than the reported ones
and give higher yields (up to 95%). In CHCl , CH Cl ,
Figure 1. Structure of the cation of 1 with ellipsoids at the 50%
probability level. Bond lengths (Å): Rh(1)−S(1), 2.3861(12); Rh(1)−
S(2), 2.3948(14); Cl(1)−Rh(1), 2.4084(11); and Rh−C, 2.179(5)−
1
2
3
2
2
2
.188(5). Bond angles (deg): S(1)−Rh(1)−S(2), 93.41(5); S(1)−
CH OH, and CH CN, the solubility of L1−L2 is good.
3
3
Rh(1)−Cl(1), 92.15(4); and S(2)−Rh(1)−C1(1), 86.84(4).
Complexes 1−4 are moderately soluble in CHCl , CH Cl , and
3
2
2
CH OH, but in CH CN, their solubility is good. The air and
3
3
moisture insensitive pale yellow liquids L1−L2 can be stored at
room temperature under ambient conditions for several
months. The solutions of complexes 1−4 made in CH CN
3
are also stable for several months under ambient conditions.
The results of elemental analyses of complexes 1−4 are
consistent with the single crystal structures (determined with
X-ray diffraction) given below.
Crystal Structures. Half-sandwich complexes of rhodium-
(
III) (1−2) and iridium(III) (3−4) appear to be formed by
5
5
chloro bridge cleavage of [(η -Cp*)RhCl(μ-Cl)] /[(η -Cp*)-
2
IrCl(μ-Cl)]2 followed by reaction with 1,2-bis(phenylthio/
selenomethyl) benzene (L1/L2) at room temperature, which is
facilitated by chloride extraction with NH PF . The single
4
6
crystals of complexes 1−4 of quality suitable for X-ray
diffraction were grown by diffusion of diethyl ether into
concentrated solutions of the complexes made in 1:4 (v/v)
methanol−acetonitrile mixtures. The crystallographic and
refinement data for 1−4 are summarized in Supporting
Information (Tables S1 and S2). The ligands L1 and L2 are
Figure 2. Structure of the cation of 2 with ellipsoids at the 30%
probability level. Bond lengths (Å): Rh(1)−Se(1), 2.4874(11);
Rh(1)−Se(2), 2.488(11); Cl(1)−Rh(1), 2.4039(16); Rh−C, and
2
.173(5)−2.188(5). Bond angles (deg): Se(1)−Rh(1)−Se(1),
96.64(2); Se(1)−Rh(1)−Cl(1), 83.32(5); and Se(2)−Rh(1)−C1(1),
93.39(5).
2
536
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center is formed through chalcogen atoms, and a chlorine atom
completes the coordination sphere. This results in an overall
three-legged piano-stool conformation.
The Rh−S bond distances in 1 [2.3861(12) and 2.3948(14)
5
Å] are normal, as their values for complexes [η -Cp*RhCl(1,1′-
(
1,2-ethanediyl)bis(3-methylimidazole-2-thione)]Cl
13
5
[2.3967(11) Å] and [η -Cp*RhCl{2-(phenylthiomethyl)-
14
pyridine}]PF [2.383(2) Å] are of the same order. The
6
Rh−Se bond lengths of the cation of 2 [2.4874(11) and
5
2
.488(11) Å] are somewhat shorter than that of complex [η -
2
15
Cp*RhCl{η -(SePPh ) N}] [2.5266(8) Å] but similar to the
values reported for the half-sandwich complex of Rh(III) with
the 2-(phenylselenomethyl)pyridine ligand [2.487(1) Å]. The
2
2
14
Figure 3. Structure of the cation of 3 with ellipsoids at the 30%
probability level. Bond lengths (Å): Ir(1)−S(1), 2.3640(7); Ir(1)−
S(2), 2.3694(8); Cl(1)−Ir(1), 2.4173(7); and Ir−C, 2.186(3)−
Rh−C (Cp* centroid) distances of complexes 1 and 2
16
[
1.806(3) and 1.810(5) Å, respectively] are normal. The
2.202(3). Bond angles (deg): S(1)−Ir(1)−S(2), 96.23(2); S(1)−
Ir−S bond distances of 3 [2.3640(7) and 2.3694(8) Å] are
within the range [2.318(1)−2.3872(10) Å] in which such bond
Ir(1)−Cl(1), 91.84(3); and S(2)−Ir(1)−C1(1), 83.82(3).
5
17
5
lengths of [η -Cp*Ir-(CO)(μ-STol)Pt(STol)(PPh )], [η -
3
2
18
5
Cp*Ir(η -ppy-S-p-tol)(H O)][OTf] , [η -Cp*Ir(4,6-di-t-
butyl-(2-methylthiophenylimino)-o-benzoquinone]-
2
2
1
9
5
[
PF ].CH Cl , [η -Cp*Ir(nBuPPh )({7-(S)PPh }-8-S-7,8-
6 2 2 2 2
20
5
C B H )],
and [η -Cp*IrCl{2-(phenylthiomethyl)-
4
2
9
10
1
pyridine}]PF6 have been reported. The Ir−Se bond distances
in 4 [2.4922(13) and 2.4547(12) Å] are consistent with the
5
values reported for [η -Cp*IrCl{2-(phenylselenomethyl)-
14
5
pyridine}]PF [2.4531(10) Å], [η -Cp*IrCl(μ-SeCOC H )-
6
6
5
2
5
22
(
[
κ -SeCOC H −)Ir(η -Cp*)] [2.445(2)− 2.495(1) Å], and
6
4
5 3
17
η -Cp*Ir(μ -Se) {PtTol(PPh )} ] [2.416(1)−2.422(1) Å]
2
3
2
5
but longer than the values reported for [η -Cp*Ir-
21
{
Se C (CO Me) }] [2.3494(7) and 2.3520(7) Å]. The
2 2 2 2
5
Ir−η -Cp*(centroid) distances (Å) in 3 [1.814(1)] and 4
1.812(8)] are normal and consistent with the values reported
Figure 4. Structure of the cation of 4 with ellipsoids at the 30%
probability level. Bond lengths (Å): Ir(1)−Se(1), 2.4922(13); Ir(1)−
Se(2), 2.4547(12); Cl(1)−Ir(1), 2.417(2); and Ir−C 2.170(9)−
[
5
23
−
for complex [(η -Cp*)Ir(phpy)Cl] [1.863 Å]. The PF6 has
been found to be involved in C−H···F secondary interactions
in all complexes 1−4, resulting in chains. In Figures 5 and 6,
they are shown for complexes 1 and 2. For the other two
complexes, noncovalent interactions are given in Supporting
Information (see Table S5 for C−H···F distances and Figures
S1 and S2).
2
.218(9). Bond angles (deg): Se(1)−Ir(1)−Se(2), 96.55(4); Se(1)−
Ir(1)−Cl(1), 83.52(8); and Se(2)−Ir(1)−C1(1), 93.23(7).
The H atoms and PF6− are omitted for clarity. The rhodium
and iridium complexes have almost similar molecular structure.
In their cations, there is a pseudo-octahedral half-sandwich
Spectral Data. The NMR spectral data of complexes 1−4
“
piano-stool” disposition of donor atoms around Rh/Ir. The
are consistent with their single-crystal structures. Two signals
5
77
1
centroid of the η -Cp* ring occupies nearly the center of a
triangular face of an octahedron. A chelate ring with the metal
observed in the Se{ H} NMR spectrum (Supporting
Information, Figures S3−S5) of each of the two complexes 2
Figure 5. Three dimensional packing framework viewing noncovalent C−H···F interactions in the crystal lattice contain PF in a polyhedral form of
6
complex 1.
2
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Figure 6. Three dimensional packing framework viewing noncovalent C−H···F interactions in the crystal lattice contain PF in a polyhedral form of
6
complex 2.
and 4 may be assigned to diastereomers. One signal in the
Se{ H} spectrum of 2 is shifted to a higher frequency (up to
Scheme 2. Transfer Hydrogenation of Carbonyls with
Glycerol
7
7
1
∼
9.0 ppm) in comparison to those of free L2. It is probably due
to the coordination of L2 with the Rh center through Se.
However, in the case of complex 4 the signals in the 77Se{ H}
NMR spectrum have been observed shifted to a lower
1
1
frequency by ∼20 ppm with respect to that of free L2. In H
and 13C{ H} NMR spectra of 1−4, signals of protons and
carbon atoms generally appear at higher frequencies relative to
those of free ligands, which coordinate with Rh and Ir in a
bidentate mode. The shifts to higher frequencies for fragment
1
glycerol is very cheap, and high recovery of this main byproduct
is not going to cut the cost of processes very significantly. The
conversion (yield up to 98%) has been found fastest in the case
of benzaldehyde with all of the present complex catalysts. The
time profile is shown in Figure 7, and conversion/yield almost
PhCH (E) (E = S or Se) [up to 8.5 ppm for carbon atoms in
2
1
3
1
C{ H) NMR spectra and 1.25 ppm for protons attached to
1
them in H NMR] imply that the coordination of ligands L1
and L2 with Rh/Ir through S/Se appears to exist in solution.
5
The signals due to the η -pentamethylcychlopentadienyl group
1
13
1
H(singlet) and C{ H} NMR spectra of complexes 1−4 were
found shifted to a lower frequency (maximum shift ∼0.25 and
2
(
5
.5 ppm respectively) with respect to those of [(η -Cp*)RhCl-
5
μ-Cl)] /[(η -Cp*)IrCl(μ-Cl)] . This may be due to the
2
2
substitution of Cl with S and Se, which have relatively lower
electronegativity.
HR-MS spectra of structurally similar complexes 1−4
−
indicate that PF6 is considerably labile and that consequently
the molecular ion peak in the mass spectra is not observed.
+
Instead, the peak of cationic fragment species M has been
observed in the spectra of all complexes, 1−4. In all mass
5
+
5
spectra, the peaks of fragment [(η -Cp*)RhCl] or [(η -
Figure 7. Time profile of catalytic TH of benzaldehyde using complex
1 and 2. Conditions: 1 mol % of catalyst, 1.0 mmol of benzaldehyde,
KOH (1.5 mmol), and 5 mL of glycerol at 120 °C, in air.
+
Cp*)IrCl] and ligands have also been observed (see
Supporting Information Figures S6−S9).
Transfer Hydrogenation of Carbonyl Compounds.
The TH of aldehydes and ketones in glycerol (also a hydrogen
donor) catalyzed with complexes 1−4 (1 mol %) in the
presence of KOH has been studied at a temperature of 120 °C
linearly increases up to 3 h, and thereafter the rate slows down.
The values of percent yields (1−10% lower than conversions)
are given in Table 1 and appear promising. The complexes (2
and 4) of Se ligand L2 are more efficient as catalysts relative to
the corresponding S analogues (1 and 3). To our knowledge,
no other example of a rhodium complex used as the catalyst for
TH with glycerol exists for comparison. Iridium complexes,
(
Scheme 2). The carbonyl compound and product both were
1
monitored with H NMR. The carbonyl compound is reduced
to the corresponding alcohol, while glycerol is dehydrogenated
to dihydroxyacetone (DHA; H NMR: δ 4.4 and 3.5 ppm) and
1
6
a
10
other products but all in low yield and difficult to separate.
including half-sandwich ones known for this purpose have
The low yield of dihydroxyacetone is not a big concern because
been used in much larger amounts (2.5 mol %) than 2 and 4 (1
2
538
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a
Table 1. Transfer Hydrogenation of Carbonyl Compounds
The catalytic efficiency varies in the order (Se, Se) > (S, S)
when other coligands are the same. The stronger electron-
donating tendency of Se than S toward the metal center
probably promotes the formation of hydride, resulting in higher
efficiency in the case of the Se ligand. In TH catalyzed with 1−
4
, the formation of the M−H-containing intermediate and the
absence of the NH group in the ligand system together suggest
26
that a conventional mechanism via alkoxide formation is most
plausible. The Rh(III) species appears to be a better catalyst
than Ir(III) (Table 1) because in the case of Ir complexes for
conversions comparable with those of Rh analogues with same
catalyst loading, the reaction time needed is somewhat more.
This observation is supported by DFT calculations.
Homogeneous vs Heterogeneous Transfer Hydro-
genation Catalysis. Formation of Ir NPs has been reported in
TH in glycerol catalyzed with iridium complexes under
microwave conditions. Such NPs have been found to show a
10a
negative effect on the catalytic process. Thus, to understand
the possibility of heterogeneous contribution to the catalytic
process, i.e., whether the present TH catalysts are homoge-
27
neous or heterogeneous, a mercury poisoning test has been
executed. With the benzaldehyde substrate in glycerol using
catalysts 2 and 4, the mercury poisoning test has been found
negative, i.e., there is no significant inhibition of product
formation (Table 2). Thus, the present catalysis appears to be
Table 2. Mercury Test for Catalytic Transfer
a
Hydrogenation
b
entry
catalyst/Hg
time (h)
conversion
c
1
1/0
5
6
5
6
99
98
92
88
d
2
1/0
c
a
3
1/400
1/400
Conditions: 2 mmol substrate, 3.0 mmol KOH, 8 mL of glycerol, and
mol % catalyst at 120 °C, in air. Isolated yield.
d
b
4
1
a
Conditions: 1 mmol benzaldehyde, 1.5 mmol KOH, 5 mL of glycerol,
b
1
c
and 1 mol % catalyst at 120 °C, in air. Determined by H NMR. Rh
complex 2. Ir complex 4.
d
mol %). As only few substrates used in the present work are
common with these reports, the scope of comparison is
somewhat narrow, making any generalization difficult. For
comparable conversions, the reaction times with 2 and 4 are
shorter than those reported with some N-heterocyclic carbene
2
8
homogeneous. The PPh poisoning test has also been used.
3
In the presence of 5 equiv of PPh , the reaction occurs with
3
only a 5% decrease in percent conversion (Table 3). The
homogeneous nature of catalysis is supported as inferred on the
basis of the Hg test.
10a
based Ir complexes
but longer in comparison to values
10b
reported for a few other Ir complexes. In comparison to Ru
6c,9
species viz. Ru(p-cymene)Cl , RuCl (TPPS) (TPPS: tris(3-
2
2
3
6
Table 3. PPh Poisoning Test for Catalytic Transfer
sulfophenyl)phosphine trisodium salt), and [Ru(η -arene)-
NHC)CO ] reported for the catalysis of TH in glycerol (in
3
a
Hydrogenation
(
3
the presence of KOH/NaOH), the present Rh and Ir
complexes are more efficient as yields are better in a shorter
time frame for benzaldehyde and relative to some Ru species,
the required catalyst loading is also low. On monitoring the TH
b
entry
catalyst/PPh3
time (h)
conversion
c
1
1/0
1/0
1/5
1/5
5
6
5
6
99
98
94
93
d
2
c
3
7
7
1
reactions catalyzed with 2 and 4 with Se{ H} NMR
spectroscopy, it is observed that the signals in the spectra
shift to higher frequency (21−28 ppm), indicating that
probably the M−Cl bond is cleaved or weakened very
significantly to make a coordination site on the metal center
available so that formation of an intermediate having a M−H
d
4
a
Conditions: 1 mmol benzaldehyde, 1.5 mmol KOH, 5 mL of glycerol,
and 1 mol % catalyst at 120 °C in air. Determined by H NMR. Rh
b
1
c
d
complex 2. Ir complex 4.
24
bond finally can takes place. The TH reactions catalyzed with
DFT Calculations. DFT calculations (see Experimental
Section for details) were performed on all four complexes 1−4.
An analysis of lowest energy configurations and frontier orbitals
leads to qualitative insight of these complexes. The HOMOs
(highest occupied molecular orbitals) of all complexes are
essentially similar and positioned primarily over the Rh or Ir
and Cp* ring. The S or Se and Cl donor atoms have only small
1
2
and 4 were also monitored with H NMR spectra. After 1 h, a
broad singlet was noticed around δ −11.7 and −12.4 ppm,
respectively. These signals are characteristic of hydride and
25
indicate the formation of an M−H bond. Thus, catalytic
reactions with the present complexes probably proceed via the
formation of a metal−hydride complex intermediate.
2
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Figure 8. Frontier molecular orbitals of complexes 1−4 and their HOMO−LUMO energy gap.
Table 4. Comparison of Selected Bond Lengths (Å) and Angles (deg) of 1−4 Determined Experimentally and Calculated by
a
DFT
1
2
3
4
bond angle/length
DFT value
bond angle/length
DFT value
bond angle/length
DFT value
bondangle/length
DFT value
M−E(1)
M−Cl(1)
M−E(2)
2.3861(12)
2.4084(11)
2.3948(14)
1.806(3)
2.541
2.429
2.543
1.820
93.07
90.23
84.35
2.4874(11)
2.4039(16)
2.4887(11)
1.810(5)
2.591
2.445
2.598
1.836
96.18
82.36
92.07
2.3640(7)
2.4173(7)
2.3694(8)
1.814(1)
96.23(2)
91.84(3)
83.82(3)
2.542
2.445
2.538
1.808
93.43
90.14
82.43
2.4922(13)
2.417(2)
2.4547(12)
1.812(8)
96.55(4)
83.52(8)
93.23(7)
2.599
2.454
2.598
1.822
95.88
81.62
90.76
a
M−C
E1−M−E2
E1−M−Cl
Cl−M−E2
93.41(5)
96.64(2)
92.15(4)
83.32(5)
86.84(4)
93.39(5)
a
a
M = rhodium or iridium, E = sulfer or selenium, C = centroid of Cp*).
share of these HOMOs. The d-orbitals of Rh(III)/Ir(III)
interacting with the π orbitals of the η -Cp* ring and the p-
Chart 2. (a) HOMO-LUMO Energy Gaps of 1−4 and (b)
Conversions (%) in 5 h for Transfer Hydrogenation of
Benzaldehyde Catalyzed with 1−4 (1 mol %) with Glycerol
5
orbital of chlorine and chalcogen atoms constitute these
HOMOs (see Figure 8 for complexes 1−4). The detailed
calculated bond length/angle parameters are given in Table 4).
The agreement between the experimentally observed bonding
parameters and those calculated by DFT is better for M−Cl
and M−Cp* (centroid). The difference between calculated and
observed M−E (E = S or Se) bond distances is of the order of
0.1 Å. The calculated and experimentally found bond angles are
also reasonably close (Table 4) except in a few cases, e.g., S1−
Ir−S2, where the difference is of the order 2°. The HOMO−
LUMO (lowest unoccupied molecular orbital) energy gap of a
29
complex may influence its chemical reactivity. This energy
gap changes between Rh and Ir species mainly due to the
variation in the energy of LUMO. The HOMO−LUMO energy
gaps between 1 and 3, and 2 and 4 sufficiently differ (Figure 8
and Supporting Information, Table S4) indicating the higher
reactivity of Rh complexes than those of Ir. The HOMO−
LUMO energy gap for complexes of the (Se, Se) ligand is
somewhat lower than those of the corresponding complexes of
Information) on metal is more negative in the case of Rh
bonded to the Se ligand. This may promote the M−H bond
and appears to be consistent with the experimental catalytic
efficiency orders, Rh > Ir and (Se, Se) > (S, S).
(
S, S) analogues (see Figure 8 and Chart 2). This may be the
cause of the somewhat higher reactivity of complexes of the
Se, Se) ligand than those of its (S, S) analogue. This is
consistent with the observed catalytic efficiency of complexes
−4 (Table 1 and Chart 2). The natural bond orbital atomic
charge (see Table S4 and Figure S10 in Supporting
(
CONCLUSIONS
■
5
1
Half-sandwich piano-stool complexes [(η -Cp*)Rh(L)Cl][PF ]
6
5
and [(η -Cp*)Ir(L)Cl][PF ] (1−4) of 1,2 bis(phenylthio/
6
2
540
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selenomethyl) benzene ligands (L = L1−L2) have been
synthesized and characterized by multinuclei NMR, HR-MS,
and X-ray crystallography. The complexes are the first examples
of half-sandwich complexes of rhodium(III)/iridium(III) with
atmosphere. 1,2-Bis(bromomethyl)benzene (2.5 mmol) dissolved in 5
cm of ethanol was added dropwise and the refluxing of the mixture
3
continued further for 1 h. After cooling to room temperature, the
3
mixture was poured into 100 cm of distilled water, neutralized with
3
dilute sodium hydroxide, and extracted with 100 cm of chloroform.
(
S, S), and (Se, Se) ligands explored for TH of carbonyl
3
The extract was washed with water (3 × 50 cm ) and dried over
compounds using glycerol as a solvent and hydrogen donor.
The 1 mol % of present Ir species required for efficient catalysis
is lower than the value 2.5 mol % reported to be optimum in
case of Ir complexes known earlier for this purpose. The 1−2
are the first examples of Rh species explored for TH in glycerol.
The yields were promising. The air and moisture stability of 1−
anhydrous sodium sulfate. Its solvent was evaporated off under
reduced pressure on a rotary evaporator, resulting in L1 as a pale
yellow oil.
1
L1: yield 0.76 g, ∼95%. H NMR (CDCl , 25 °C vs Me Si) δ
3
4
(ppm): 4.21 (s, 4H, H
, H12), 7.11−7.18 (m, 4H, H7−10), 7.19−7.25
5
1
3
1
(
(
1
m, 6H, H1−2, H15−16), 7.26−7.30 (m, 4H, H , H ). C{ H} NMR
3
14
CDCl , 25 °C vs Me Si) δ (ppm): 36.7 (C , C ), 126.6 (C , C ),
4
is an additional advantage. The catalytic efficiencies follow the
3
4
5
12
1
16
27.7(C , C ), 128.9 (C , C ), 130.4 (C , C ), 130.6 (C , C ), 135.6
3
14
2
15
7
10
8
9
orders Rh > Ir and (Se, Se) > (S, S), which are corroborated by
DFT studies. There is no NP formation in the catalytic process
which appears to be homogeneous. The formation of the M−H
bond in the absence of the NH group suggests a conventional
mechanism via alkoxide formation for TH.
−
1
(
C , C ), 136.3 (C , C ). IR (KBr, cm ): 3020 (m; νC−H (aromatic)),
4 13 6 11
2
940 (s; νC−H (aliphatic)), 1475 (m; ν
), 750 (m;
CC (aromatic)
νC−H(aromatic)).
Synthesis of L2. Diphenyldiselenide (0.6245, 2.0 mmol) dissolved
3
in 30 cm of ethanol was treated with a solution (made in 5% NaOH)
of NaBH4 (0.149 g, 4.0 mmol) (added dropwise) under N2
atmosphere until it became colorless due to the formation of PhSeNa.
1,2-Bis(bromomethyl)benzene (2.0 mmol) dissolved in 5 cm of
EXPERIMENTAL SECTION
Physical Measurements. The H, C{ H}, and Se{ H} NMR
spectra were recorded on a 300 MHz NMR spectrometer at 300.13,
5.47, and 57.24 MHz, respectively. The chemical shifts are given in
■
3
1
13
1
77
1
ethanol was added to the colorless solution with constant stirring. The
reaction mixture was stirred further for 2−3 h and poured into 100
7
3
3
cm of ice-cold distilled water and extracted with CHCl (5 × 40 cm ).
ppm relative to known standards. Yields refer to isolated yields of
3
3
1
The extract was washed with water (3 × 50 cm ) and dried over
anhydrous sodium sulfate. Its solvent was evaporated off under
compounds which have a purity ≥95% [established by H NMR]. IR
−
1
spectra in the range 4000−400 cm were recorded on a FT−IR
spectrometer as KBr pellets. The C, H, and N analyses were carried
out with a C, H, and N analyzer. For single crystal structures, the data
were collected on a CCD diffractometer using Mo Kα (0.71073 Å)
reduced pressure on a rotary evaporator, resulting in L2 as a pale
1
yellow oil. Yield: 0.75 g, ∼90%. H NMR (CDCl , 25 °C vs Me Si) δ
3
4
(
(
(
1
1
ppm): 4.18 (s, 4H, H , H ), 7.02−7.09 (m, 4H, H ), 7.18−7.27
7−10
13 1
5
12
30
m, 6H, H , H
), 7.41−7.44 (m, 4H, H , H ). C{ H} NMR
radiation at 298(2) K. The software SADABS was used for
absorption correction (if needed) and SHELXTL for space group,
structure determination, and refinements. Hydrogen atoms were
included in idealized positions with isotropic thermal parameters set at
.2 times that of the carbon atom to which they are attached in all
cases. High resolution mass spectral measurements were performed
with electron spray ionization (10 eV, 180 °C source temperature, and
sodium formate as reference compound) taking samples in CH CN.
All reactions have been carried out in glassware dried in an oven, under
ambient conditions except for the synthesis of L1 and L2. The
commercial nitrogen gas has been used after passing it successively
through traps containing solutions of alkaline anthraquinone, sodium
dithionite, alkaline pyrogallol, concentrated H SO , and KOH pellets.
Nitrogen atmosphere if required was created using Schlenk techniques.
Chemical and Reagents. Diphenyldiselenide, thiophenol,
1−2
15−16
3
14
CDCl , 25 °C vs Me Si) δ (ppm): 29.7 (C , C ), 127.4 (C , C ),
3
4
5
12
1
16
3
1
29.0 (C , C ), 130.5 (C , C ), 130.6 (C , C ), 131.6 (C , C ),
2
15
7
10
8
9
4
13
77
1
33.8 (C , C ), 136.5 (C , C ). Se{ H} NMR (CDCl , 25 °C vs
3
14
6
11
3
−
1
Me Se) δ (ppm): 358.8, IR (KBr, cm ): 3060 (m; νC−H (aromatic)),
1
2
2
934 (s; νC−H (aliphatic)), 1479 (m; ν
), 736 (m;
CC (aromatic)
νC−H (aromatic)).
5
Synthesis of Complexes 1−2. The solid [(η -Cp*)RhCl(μ-Cl)]
2
3
(
0.05 g, 0.1 mmol) and ligand L1/L2 (0.2 mmol) dissolved in
3
CH OH (15 cm ) were mixed together and the mixture stirred for 8 h
3
at room temperature. The resulting orange solution was filtered, and
3
the volume of the filtrate was reduced (∼7 cm ) with a rotary
evaporator. It was mixed with solid NH PF (0.032 g, 0.2 mmol), and
4
6
2
4
the resulting orange colored microcrystalline solid was filtered, washed
3
with ice-cold 10 cm of CH OH, and dried in vacuo. Single crystals of
3
1
and 2 were obtained by the diffusion of diethyl ether into their
NaBH , 1,2-bis(bromomethyl)benzene, and ammonium hexafluor-
4
3
solutions (4 cm ) made in a mixture (1:4) of CH OH and CH CN.
ophosphate procured from Sigma−Aldrich (USA) were used as
3
3
5
32
5
33
Compound 1: yield 0.126 g, ∼85%. Anal Calcd for C H ClRhS ·
received. The [(η -Cp*)RhCl(μ-Cl)] and [(η -Cp*)IrCl(μ-Cl)]
30 33
2
2
2
1
[
PF ]: C, 48.62; H, 4.49; found, C, 48.59; H, 4.48. Mp 260 °C. H
6
were prepared according to literature methods. All of the solvents were
34
NMR (CD CN, 25 °C vs Me Si) δ (ppm): 1.49 (s, 15H, H of
dried and distilled before use by standard procedures. The common
reagents and chemicals available locally were used.
3
4
Me(Cp)), 4.29−4.35 (m, 4H, H , H ), 7.06−7.21 (m, 4H, H7−10),
5
12
7
.35−7.56 (m, 6H, H , H
), 7.58−7.76 (m, 4H, H , H ).
DFT Calculations. All DFT calculations were carried out at the
Department of Chemistry, Supercomputing Facility for Bioinformatics
and Computational Biology, IIT Delhi, with the Gaussian-09
1−2
15−16
3
14
1
3
1
C{ H} NMR (CDCl , 25 °C vs Me Si) δ (ppm): 8.14 (C of
3
4
Me(Cp*), 44.6 (C
, C12), 101.3 (C of Cp*), 127.4 (C
, C15), 130.1 (C , C10), 131.3 (C , C
, C11). HR-MS (CH CN) [M] (m/z) = 595.0776;
calulated value for C30 33ClRhS = 595.0762 (δ: −2.5 ppm). IR (KBr,
cm ): 3054 (m; νC−H (aromatic)), 2972 (s; νC−H (aliphatic)), 1442 (m;
, C16),
), 132.2
9
5
1
35
program with an immediate objective of identifying the reactivity
order in the present series of compounds. The geometry of complexes
128.6(C
3
, C14), 129.2 (C
, C13), 133.8 (C
2
7
8
+
(C
4
6
3
36
37
1
to 4 was optimized at the M06 level using a LANL2DZ basis set
H
2
−
1
for metal and chalcogen atoms and 6-31G* basis sets for C, H, and Cl.
Natural bond orbital (NBO) analysis of atomic charges has been done
ν
CC (aromatic)), 831 (s; νP−F), 764 (m; νC−H(aromatic)).
3
6
for all compounds 1 to 4 by using the M06 functional. All
calculations have been carried out in gas phase and at 298.15 K.
Geometry optimizations were carried out without any symmetry
restriction with X-ray coordinates of the molecule. Harmonic force
constants were computed at the optimized geometries to characterize
the stationary points as minima. The molecular orbital plots were
made using the Chemcraft program package (http://www.
chemcraftprog.com).
Compound 2: yield 0.150 g, ∼90%. Anal Calcd for C30H33ClRhSe ·
2
1
[PF
NMR (CDCl
Me(Cp)), 4.25−5.23 (m, 4H, H
7.52−7.61 (m, 6H, H1−2, H15−16), 8.21−8.26 (m, 4H, H
]: C, 43.16; H, 3.98; found, C, 43.65; H, 3.92. Mp 242 °C. H
6
, 25 °C vs Me
Si) δ (ppm): 1.38 (s, 15H, H of
, H12), 7.20−7.30 (m, 4H, H7−10),
, H14).
Si) δ (ppm): 8.03 (C of
Me(Cp*), 38.2 (C , C ), 100.9 (C of Cp*), 129.5 (C , C ), 129.7
3
4
5
3
13
1
C{ H} NMR (CDCl , 25 °C vs Me
3 4
5
12
1
16
(C , C ), 129.9 (C , C ), 131.4 (C , C ), 132.1 (C , C ), 133.7 (C ,
C ), 135.3 (C , C ). Se{ H} NMR (CDCl , 25 °C vs Me Se) δ
(ppm): 294.7 (Se1), 367.3 (Se2) HR-MS (CH CN) [M] (m/z) =
2
15
7
10
8
9
4
13
3
7
7
1
Synthesis of L1. Sodium hydroxide (0.224 g, 6 mmol) dissolved in
1
4
6
11
3
2
3
+
5
cm of water was added dropwise to thiophenol (0.5 mL, ∼5 mmol)
3
3
taken in 50 cm of dry ethanol and refluxed for 0.5 h under N
690.9632; calulated value for C H ClRhSe = 960.9652 (δ: 2.9 ppm).
2
30 33
2
2
541
dx.doi.org/10.1021/om500149n | Organometallics 2014, 33, 2535−2543

Organometallics
Article
−
1
IR (KBr, cm ): 3063 (m; νC−H (aromatic)), 2979 (s; νC−H (aliphatic)), 1443
material is available free of charge via the Internet at http://
pubs.acs.org.
(
m; ν
), 829 (s; νP−F), 774 (m; ν
).
CC (aromatic)
C−H(aromatic)
5
Synthesis of Complexes 3−4. The solid [(η -Cp*)IrCl(μ-Cl)]
2
(
0.05 g, 0.1 mmol) was mixed with a ligand out of L1−L2 (0.2 mmol)
AUTHOR INFORMATION
Corresponding Author
3
■
dissolved in CH OH (15 cm ) and the mixture stirred for 10 h at
3
room temperature. The resulting yellow solution was filtered. After a
work up as described for 1−2, single crystals of 3−4 were obtained by
the diffusion of diethyl ether into their solution (4 cm ) made in a
mixture (1:4) of CH OH and CH CN.
Compound 3: yield 0.136 g, ∼82%. Anal Calcd for C H ClIrS ·
PF ]: C, 43.39; H, 4.01; found, C, 43.53; H, 4.14. Mp 260 °C. H
6
*Tel: +91 11 26591379. Fax: +91 11 26581102. E-mail:
aksingh@chemistry.iitd.ac.in.
3
3
3
Notes
30
33
2
1
The authors declare no competing financial interest.
[
NMR (CD CN, 25 °C vs Me Si) δ (ppm): 1.39 (s, 15H, H of
3
4
Me(Cp)), 4.31−4.38 (m, 4H, H , H ), 7.15−7.28 (m, 4H, H7−10),
ACKNOWLEDGMENTS
5
12
■
7
.31−7.57 (m, 6H, H , H
), 7.91−8.21 (m, 4H, H , H ).
1
−2
15−16
3
14
We thank Professor B. Jayaram for computational facilities. We
also thank Department of Science and Technology, New Delhi
India) for the following: research project no. SR/S1/IC-40/
2010, partial financial assistance (under the FIST program) for
the single crystal X-ray diffractometer and mass spectral
facilities at IIT Delhi. O.P. and H.J. thank University Grants
Commission (India) and K.N.S. Council of Scientific and
Industrial Research (India) for the award of Junior/Senior
Research Fellowships.
1
3
1
C{ H} NMR (CDCl , 25 °C vs Me Si) δ (ppm): 8.09 (C of
3
4
Me(Cp*), 35.8 (C , C ), 95.0 (C of Cp*), 129.0 (C , C ), 129.8 (C ,
5
12
1
16
3
(
C ), 131.7 (C , C ), 132.3 (C , C ), 133.6 (C , C ), 134.1 (C , C ),
1
4
2
15
7
10
8
9
4
13
+
1
35.8 (C , C ). HR-MS (CH CN) [M] (m/z) = 685.1331; calulated
value for C H ClIrS = 685.1326 (δ: −0.6 ppm). IR (KBr, cm ):
092 (m; ν
6 11 3
−1
30
33
2
3
), 2957 (s; ν
), 1444 (m;
C−H (aromatic)
C−H (aliphatic)
ν
), 831 (s; νP−F), 747 (m; νC−H (aromatic)).
Compound 4: yield 0.157 g, ∼85%. Anal Calcd for C H ClIrSe ·
CC (aromatic)
30
33
2
1
[
PF ]: C, 38.99; H, 3.60; found, C, 38.79; H, 3.82. Mp 242 °C. H
6
NMR (CDCl , 25 °C vs Me Si) δ (ppm): 1.30 (s, 15H, H of
3
4
Me(Cp)), 4.40−5.43 (m, 4H, H , H ), 7.17−7.31 (m, 4H, H7−10),
5
12
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■
7.45−7.63 (m, 6H, H , H
), 8.16−8.18 (m, 4H, H , H ).
1
−2
15−16
3
14
1
3
1
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3
4
Me(Cp*), 36.2 (C , C ), 93.9 (C of Cp*), 129.3 (C , C ), 129.9 (C ,
5
12
1
16
2
(
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5
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10
8
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4
13
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14
7
7
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1
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3
0
33
2
−1
cm ): 3075 (m; νC−H (aromatic)), 2942 (s; νC−H (aliphatic)), 1443 (m;
(
ν
), 830 (s; νP−F), 739(m; νC−H (aromatic)).
Procedure for the Catalytic Transfer Hydrogenation of
CC (aromatic)
(
Carbonyl Compounds. A capped round-bottomed flask containing a
(
stirrer bar was charged with the solution of a substrate (2 mmol) made
3
in glycerol (8 cm ), KOH (3.0 mmol), and a complex from 1 to 4 (1
1
(
mol %). The mixture was heated at 120 °C for an appropriate time as
given in Table 1. After completion of the reaction, the reaction mixture
3
was cooled to room temperature and mixed with water (20 cm ). The
3
mixture was extracted with diethyl ether (3 × 20 cm ), and the solvent
2
from the extract was removed on a rotary evaporator resulting in a
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3
3
column (∼8 cm in length). The column was washed with ∼50 cm of
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1
3
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1
products were calculated and reported in Table 1. The H NMR
authenticating these products are reported in Supporting Information
(
Figures S11−S17).
Hg Poisoning Test. An excess of Hg (Hg/Rh/Ir: 400:1) was taken
in a reaction flask. The TH reaction of benzaldehyde (1.0 mmol) with
glycerol (5 mL) using 2 or 4 (1 mol %) as catalyst was carried out in
1
the flask under optimum conditions. An ∼92% conversion (with H
NMR) was observed after 5−6 h of reaction.
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3
4
916. (c) Fujii, A.; Hashiguchi, S.; Uematsu, N.; Ikariya, T.; Noyori, R.
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ASSOCIATED CONTENT
■
(
*
S
Supporting Information
(
2
Crystal data and refinement parameters, bond lengths and
angles, HOMO−LUMO energy gap and partial charges,
distances of noncovalent interactions, noncovalent interactions,
NMR and mass spectra, and CIF (CCDC numbers: 971499,
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9
71500, 971501, and 971502 (for 1 to 4, respectively). This
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dx.doi.org/10.1021/om500149n | Organometallics 2014, 33, 2535−2543

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dx.doi.org/10.1021/om500149n | Organometallics 2014, 33, 2535−2543